In a large bowl combine the ground turkey, egg, salt, black pepper, dried parsley, poultry seasoning, and cold milk; stir until just combined—mixture may be slightly wet.
Preheat your cooking surface: a cast iron skillet, electric griddle, or outdoor grill.
Divide the mixture into 4 even portions and shape into patties. Cut the cold butter into thin half-tablespoon pats and press one pat into the center of each patty, sealing the meat around the butter.
If the patties feel loose, chill them in the freezer for 10–15 minutes to firm up.
Lightly spray the preheated surface with nonstick spray. Place patties on the surface and cook about 4 minutes per side, until browned and the internal temperature reaches 165°F (74°C).
When the burgers are nearly done, place a slice of American cheese on each patty and cover with a lid or foil for about 1 minute to melt the cheese.
Remove the patties from the heat, tent with foil, and let rest for about 10 minutes.
Assemble burgers on buns and add any desired toppings and condiments, then serve.